It is generally known to apply a set point signal applied to a control system which compares the set point signal with a process output signal to control an actuator acting on the magnitude to be controlled which is measured by the process output signal. This control system may conventionally include proportional, integral and/or derivative channels. It may also have a time-delay function.
Even with these optimally controlled actions this known method cannot always limit as much as desirable errors between the process output signal and the set point signal, especially if the values of a variable magnitude other than the magnitude to be controlled, called an "external" magnitude hereinafter, are imposed on the process independently of the set point signal.
There are known applications of this general method to the specific instance where the magnitude to be controlled is the feed water level in a steam generator associated with a pressurized water reactor in a nuclear power station. To be more precise the, FR 2 312 728 describes a steam generator feed water inlet flowrate control device. This device can provide good control of the feed water flowrate at low power outputs.
FR 2 426 933 discloses another steam generator water level control method. This method achieves good control of the water level during the transient stage when the power station is commissioned.
These two documents concern relatively stable operation of the steam generator. This is the situation in electrical power stations which operate under steady state conditions and usually at virtually 100% of the rated output. However is very different from an industrial process in which the operation of these steam generators is subject to transient conditions manifested in major variations in an "external" magnitude imposed on the process. Such variations affect the flowrate of steam required from the generator in the case of variations in the power required from the power station, for example. In this case, the known control devices respond by opposing induced variations that these variations in the external magnitude tend to impose on a magnitude to be controlled, such as the water level in the steam generator. However, they are unable to prevent controlled.